Structure and elasticity of CaC2O5 suggests carbonate contribution to the seismic anomalies of Earth’s mantle

Knowledge of carbonate compounds under high pressure inside Earth is key to understanding the internal structure of the Earth, the deep carbon cycle and major geological events. Here we use first-principles simulations to calculate the structure and elasticity of CaC2O5-minerals with different symmetries under high pressure. Our calculations show that CaC2O5-minerals represent a group of low-density low-seismic-wave velocity mantle minerals. Changes in seismic wave velocity caused by the phase transformation of CaC2O5-Cc to CaC2O5-I\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{4}$$\end{document}4¯2d (CaC2O5-C2-l) agree with wave velocity discontinuity at a depth of 660 km in the mantle transition zone. Moreover, when CaC2O5-Fdd2 transforms into CaC2O5-C2 under 70 GPa, its shear wave velocity decreases by 7.4%, and its density increases by 5.8%, which is consistent with the characteristics of large low-shear-velocity provinces (LLSVPs). Furthermore, the shear wave velocity of CaC2O5-I\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{4}$$\end{document}4¯2d is very similar to that of cubic Ca-perovskite, which is one of the main constituents of the previously detected LLSVPs. Therefore, we propose that CaC2O5 and its high-pressure polymorphs may be a main component of LLSVPs.

impact of CaC 2 O 5 polymorphs on the mantle or their structural stability and elastic properties.
In this study, to understand the possible model of phase transformation of those CaC 2 O 5 polymorphs and its impact on the mantle structure, composition and deep carbon cycling, the lattice parameters, electronic properties, and elasticity of the 6 polymorphs of CaC 2 O 5 (hereafter referred to as CaC 2 O 5 s), including CaC 2 O 5 -Cc, CaC 2 O 5 -Fdd2, CaC 2 O 5 -C2-l, CaC 2 O 5 -C2, CaC 2 O 5 -I 42d and CaC 2 O 5 -Pc, were calculated by first-principles simulation for their respective stable pressure ranges.First-principles calculations have been successfully applied to geosciences to understand mineral properties such as structural, elastic properties, electronic properties, etc., under high pressure and temperature [6][7][8][9][10][11] .Our results provide insights into the presence of various structural phases of CaC 2 O 5 in the mantle and their effects on the deep mantle.

Results and discussion
Structural stability of CaC 2 O 5 -I 42d under high pressure CaC 2 O 5 -I 42d was first synthesized in the laboratory under two pressures of 34 GPa and 45 GPa 4 .However, Sagatova et al. proposed 5 that this mineral can be stable at 25 to 50 GPa from the first-principles simulations, so its stability range is unclear.The thermodynamic properties of minerals are usually evaluated by analyzing their phonon frequencies across the Brillouin Zone [12][13][14] .Therefore, the phonon dispersion along selected high-symmetry points in the Brillouin zone of CaC 2 O 5 -I 42d was calculated at 30 to 100 GPa (see Supplementary Fig. 1).Our calculated results show that lattice vibrations produce negative values in the Brillouin region under pressures between 0 and 34 GPa, which indicates that CaC 2 O 5 -I 42d is unstable in this pressure range 15 .With increasing pressure, the structure shows thermodynamic stability between 34 and 100 GPa.
To further verify the stability of CaC 2 O 5 -I 42d, we calculated the Mulliken population to explore whether the electronic properties of CaC 2 O 5 -I 42d undergo mutations at high pressure (see Supplementary Table 1).The band spilling parameter for spin component 1 was 0.69%, indicating that our calculated results are reasonable and reliable 16 .
The Mulliken population analysis of CaC 2 O 5 -I 42d at pressures of 34 GPa, 45 GPa, 70 GPa, 85 GPa, and 100 GPa showed that the population of all three C-O bonds was greater than 0.5, with a maximum difference of only 5.17% at different pressures.This result indicates that these bonds are covalent.However, the population values of all three Ca-O bonds are less than 0.1.The population of Ca-O a and Ca-O b bonds changes significantly with pressure, while the population of Ca-O c bonds remains relatively unaffected by pressure.This result indicates that these bonds are ionic.Furthermore, there was no abrupt change in the atomic electronegativity or bond denstiy from 34 to 100 GPa, indicating the stability of CaC 2 O 5 -I 42d.In summary, the phonon dispersion and Mulliken population results show that CaC 2 O 5 -I 42d maintains structural stability under 34-100 GPa.

Lattice parameters and density
The lattice parameters (a, b, and c) of CaC 2 O 5 s under high pressure were calculated (see Supplementary Fig. 2).To elucidate the differences in the lattice parameters of CaC 2 O 5 , 2 × 2 × 1 supercells of CaC 2 O 5 -C2 and CaC 2 O 5 -Pc and a 2 × 1 × 1 supercell of CaC 2 O 5 -Cc were built to ensure consistent atomic numbers in CaC 2 O 5 crystal cells.The lattice constants of CaC 2 O 5 s linearly decrease with increasing pressure (R 2 > 0.978).Notably, under the same pressure, the lattice parameters, a and b, of CaC 2 O 5 -I 42d and CaC 2 O 5 -C2-l are very close in value, and the ratio of the lattice parameter, c, of the two polymorphs is constant at 0.8 (±0.05).
The densities of all 6 calculated CaC 2 O 5 polymorphs under high pressure are listed in Fig. 1.Moreover, the densities of the main mineral phases of the lower mantle, such as CaSiO 3 perovskite 17 , MgSiO 3 perovskite 18 , FeOOH 19 , δ-AlOOH20 , and the newly discovered CaO 3 21 under high pressure, and the density of the Preliminary Reference Earth Model (PREM) 22 are also listed in Fig. 1.
The densities of CaC 2 O 5 s increase with increasing pressure, but all the densities are lower than those of the PREM model.Generally, lowdensity minerals have difficulty entering the deep mantle.However, several low-density minerals, such as δ-AlOOH 20 and CaO 3

21
, have also been discovered in the lower mantle.Therefore, due to FeOOH 18 and silicate minerals with densities higher than those of the PREM are widely present in the mantle, CaC 2 O 5 s may play an essential role in regulating and neutralizing the mantle density in the Earth's lower mantle.
As pressure increases, some clear density transitions occur between different polymorphs of CaC 2 O 5 .First, there is a density increase of 8.81%-7.37%from 20 GPa to 30 GPa when CaC 2 O 5 -Cc transforms into CaC 2 O 5 -C2-l at approximately 660 km, which is very consistent with the increase in density at 660 km in the PREM model, indicating that this structural phase transformation may be one of the reasons for the changes in density here.Second, when CaC

Elasticity
The elastic parameters of minerals and their dependence on pressure are essential in Earth science for understanding processes ranging from brittle failure to flexure to the propagation of elastic waves.Seismic observations reveal the structure of the Earth, including the radial (one-dimensional) profile, lateral heterogeneity, and anisotropy, which are primarily determined by the elastic parameters of minerals and their dependence on pressure and temperature 23 .Therefore, to understand the geological properties of CaC 2 O 5 , we calculated the elastic constants of CaC 2 O 5 s under high pressures, as shown in Fig. 2.
Based on a theorem for determining the elastic stability of minerals 24 , the elastic stability of CaC 2 O 5 s was investigated, and the results showed that the 6 polymorphs maintain elastic stability within their calculated pressure ranges.Although the space groups of CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d are different, their elastic constants show good consistency.Within the same pressure range, C 11 of ) Pressure (GPa) The elastic modulus is an important parameter for describing the physical and chemical properties of minerals.In crystalline systems, assuming that the arrangement direction is random, the bulk modulus (K) and shear modulus (G) can be obtained by the Voigt, Reuss, and Hill formulas 25 , and the Hill modulus is used here 26  Generally, understanding the composition, physical state, and structure of the Earth's interior mainly relies on observing the seismic wave velocity.Quantified seismic velocity data link seismic observations and geological characteristics, such as modal mineral composition and velocity structure.Laboratory measurements and computer simulations of the seismic wave velocity of minerals under appropriate pressure and temperature conditions have long been used to interpret the velocity structure of the Earth, combined with the results of  5 were compared (the space group symbol is shown in the figure).Pc, C2, Cc, and C2-l belong to the monoclinic system, with 13 independent elastic constants; Fdd2 belongs to the orthorhombic system, with 9 independent elastic constants; I 42d belongs to the tetragonal system, with 6 independent elastic constants.Elastic constants in the same direction are represented by the same color.large-scale geophysical imaging techniques 27 .As a potentially important component of the deep Earth, the seismic wave velocity of CaC 2 O 5 with different symmetries under high pressure is highly important for understanding the structure and composition of the mantle.Therefore, we can calculate the shear-wave (V S ) and compressional-wave (V P ) velocities of CaC 2 O 5 s (see Methods) in Fig. 3c, d

Relationship between CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d
As discussed above, some of the characteristics of CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d show good consistency.For example, under the same pressure, the lattice parameters a and b, and the density of these two polymorphs are almost the same (the difference is less than 0.02), and the lattice parameter c maintains a constant ratio of 0.8 (Supplementary Fig. 2).The differences in the lattice parameter c of the two polymorphs came from the differences in the bond angle β in their cell structure, namely, 125°in CaC 2 O 5 -C2-l and 90°in CaC 2 O 5 -I 42d.The V P and V S of CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d also exhibit good consistency under the same pressure (the differences in V P and V S are less than 0.08 km s −1 and 0.03 km s −1 , respectively) (Fig. 4).
To further verify the relationship between the CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d, we calculated the energy band structure and the density of states under high pressure from CaC 2 O 5 s.
The band gap of CaC 2 O 5 s varies between 5.108 eV and 7.701 eV under the explored pressure in this work, indicating their insulating properties 28 (Fig. 4a).The band gaps of CaC  4b, c).The difference in the orbitals contributions is less than 0.3%.The characteristics of the DOS of CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d are primarily determined by the electrons distributed in the p orbitals.At 40 GPa, the p orbitals contribute approximately 62.94% and 63.12%, respectively, to the total DOS.Moreover, the remaining contributions are attributed to the s orbitals (23.67% and 23.48%) and the d orbitals (13.39% and 13.40%).
The cell parameters, electronic properties, elasticity, and wave velocity of CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d under high pressure are almost identical, and the trends with pressure are also almost identical.Hence, these two phases are the same thing.

Effects of CaC 2 O 5 s on the structure of the mantle
To explore the effects of CaC 2 O 5 polymorphs on the structure of the mantle, we compared our calculations with the wave velocities of sseveral common mantle minerals (Fig. 5), including wadsleyite 29 , ringwoodite 30 , cubic Ca-Pv 31 , MgSiO 3 perovskite 18 and CaSiO 3 perovskite 17 under high pressure.Moreover, the seismic wave velocity of the PREM 22 is also presented here.
The wave velocities of CaC 2 O 5 s under the explored high pressure in this work are lower than those of the PREM model.The wave velocities of CaC 2 O 5 s are also lower than those of common minerals in the  with that of Earth's mantle according to the PREM 22 , the black thin dotted line, the gray area, and the black arrows represent the boundaries at which the phase transition begins, the region where the phase transition occurs, and which two types of CaC 2 O 5 undergo the phase transition 5 , respectively.mantle, including wadsleyite, ringwoodite, MgSiO 3 perovskite, and CaSiO 3 perovskite; therefore CaC 2 O 5 s are a set of low-velocity minerals and may be useful for understanding the origin of the low-velocity layer in the mantle.
From depths of 300 km to 1662 km, the wave velocity increases with pressure.Two significant velocity surges are observed during the phase transformation among the CaC 2 O 5 polymorphs: one is near 660 km depth when the CaC 2 O 5 -Cc structure transforms into CaC 2 O 5 -I 42d (C2-l), and the other is near 1435 km depth when the CaC 2 O 5 -I 42d structure transforms into CaC 2 O 5 -Fdd2.Afterwards, from depths of 1662 km to 2500 km, there is a sudden drop in wave velocity when CaC 2 O 5 -Fdd2 transforms into CaC 2 O 5 -C2 and CaC 2 O 5 -Pc near a depth of 1662 km.
CaC 2 O 5 -Cc transforms into CaC 2 O 5 -I 42d (CaC 2 O 5 -C2-l) at 21 GPa (0 K) to 29 GPa (2500 K) 5 , leading to an increase in V S of 13.84% and an increase in V P of 9.18% (Fig. 5).This phase transformation depth matches the depth of the 660 km discontinuity zone of the mantle 32,33 .The general view of the seismic wave velocity discontinuity at 660 km in the mantle transition zone is caused by the postspinel phase transformation of ringwoodite (ringwoodite (Mg, Fe)2SiO 4 → perovskite (Mg, Fe)SiO 3 + ferropericlase (Mg, Fe)O) 34 .The density of the PREM model increases from 3.99 g cm −3 to 4.38 g cm −3 , V S increases from 5.57 km s −1 to 5.95 km s −1 , and V P increases from 10.26 km s −1 to 10.75 km s −1 at 660 km, and the increase in density, V S and V P are 0.37 g cm −3 , 0.38 km s −1 , and 0.49 km s −1 , respectively.When CaC 2 O 5 -Cc transforms into CaC 2 O 5 -I 42d (CaC 2 O 5 -C2-l) at a depth of 660 km, its density increases from 3.38 g cm −3 to 3.67 g cm −3 by 0.29 g cm −3 , V S increases from 4.25 km s −1 to 4.81 km s −1 by 0.55 km s −1 , and V P increases from 9.28 km s −1 to 8.50 km s −1 by 0.78 km s −1 .The CaC 2 O 5 and PREM have very similar densities, and the shear-wave velocity increases at 660 km.Therefore, we propose that the transformation of CaC 2 O 5 -Cc to CaC 2 O 5 -I 42d (CaC 2 O 5 -C2-l) may also be one of the origins of wave velocity discontinuity at a depth of 660 km in the mantle transition zone.
Studying the wave velocity structure of the large low-shearvelocity provinces (LLSVPs) in the lower mantle is highly important for understanding the dynamic evolution process of Earth's interior.LLSVPs play a crucial role in regulating heat flow from the core, facilitating the exchange of matter and energy between the Earth's deep layers and influencing continental evolution, surface resources, and the environment.LLSVPs are regions with shear wave velocities (V S ) several percent lower than those of the surrounding mantle [35][36][37][38] .There are two large low shear wave velocity bodies that extend thousands of kilometres horizontally and hundreds of kilometres vertically on the Core-Mantle boundary below Africa and the Pacific Ocean [38][39][40] .There are many controversies about the origin and evolution of these areas 41 .Thomson et al. proposed a cubic Ca-Pv perovskite that can perfectly match the characteristics of low-shear-velocity minerals in LLSVPs 31 (Fig. 5) and is considered an interpretation of the origin of LLSVPs.Here, our calculated V S of CaC 2 O 5 -I 42d is almost the same as that of cubic Ca-Pv perovskite, and when CaC 2 O 5 -Fdd2 transforms into CaC 2 O 5 -C2 as the pressure increases, its V S decreases by 7.4% (Fig. 5).It's worth noting that CaC 2 O 5 -I 42d was synthesized via hightemperature and high-pressure experiments at 34 GPa and 45 GPa by the reaction of CaCO 3 + CO  LLSVPs not only exhibit low-velocity velocities that are several percent lower than the those of the surrounding mantle but also exhibit higher density than the surrounding components [42][43][44] .When CaC 2 O 5 -Fdd2 transforms into CaC 2 O 5 -C2, its density increases by 5.8% (Fig. 1).This trend allows CaC 2 O 5 transformation to more perfectly match the characteristics of LLSVPs.Therefore, we determine that CaC 2 O 5 -I 42d and its high-pressure polymorphs are very likely to be in the interior of the LLSVPs region and may serve as a supplement to the Ca-Pv component in LLSVPs or may be one of the main components in LLSVPs.
Based on our calculated density, wave velocity of CaC 2 O 5 s, probable phase transformation, and reactions of CaC 2 O 5 in the mantle, a model of the deep carbon cycle is proposed (Fig. 6).Calcium carbonate (CaCO 3 ) and CO 2 can reach the deep Earth through subduction plates.The discovery of calcium carbonate inclusions in superdeep diamonds 45 and fluid CO 2 wrapped in diamonds 46 also confirmed the occurrence of calcium carbonate and CO 2 in the deep mantle.Therefore, CaC 2 O 5 and its high-pressure polymorphs could be produced by the reaction CaCO 3 + CO 2 → CaC 2 O 5 at different depths 4 in the mantle.CaC 2 O 5 -Cc in the shallow mantle transforms into CaC 2 O 5 -I 42d at 660 km depth, causing an anomalous increase in wave velocity in the mantle transition zone.Meanwhile, CaC 2 O 5 -Fdd2 in the deep mantle may partially transforms into CaC 2 O 5 -C2 with increasing pressure, leading to an increase in mantle density but a decrease in shear-wave velocity, which is one of the possible reasons for explaining the origin of LLSVPs [42][43][44] .During these reactions, CaC 2 O 5 reacts with minerals such as (Mg, Ca)SiO 3 perovskite and ferropericlase (MgO) in the lower mantle and produce magnesium carbonate (MgCO 3 ), calcium carbonate (CaCO 3 ), calcium silicate(CaSiO 3 ), and CO 2 3,5 . The more stable MgCO 3 and CaSiO 3 remain in the lower mantle as the main carbonate minerals 3 and the main minerals in LLSVPs 31 , respectively.CaCO 3 generated by the reaction may repeat the process of reacting with CO 2 to generate CaC 2 O 5 or decompose into CO 2   internal dynamic processes.For example, mantle plumes, which are high-temperature environments, facilitate the rise of gaseous carbon dioxide back to the upper mantle and the bottom of the lithosphere.Therefore, CaC 2 O 5 exhibits important effects on the structure and composition of the mantle as well as the Earth's deep carbon cycle.Our work provides different insights for understanding the origin of the wave velocity discontinuity at the depth of 660 km in the mantle transition zone and at the LLSVPs.It also sheds light on the deep carbon cycle model of the Earth.

Methods
The density functional theory (DFT) calculations First principles calculations are performed using density functional theory 47,48 with the plane-wave pseudopotential.The calculations are implemented in the CASTEP code 49 .The generalized gradient approximation (GGA) with PBE parameterization 50 is employed to describe exchange-correlation interactions.OTFG ultrasoft pseudopotentials 51,52 are utilized to model electron-ion interactions with a plane-wave energy cutoff set in Table 1.A Monkhorst Pack grid of k-points was employed to sample the Brillouin zone.Table 1 presents the plane-wave energy cutoff and k-points settings for the various CaC 2 O 5 phase states.A convergence criterion of 5 × 10 −7 a.u.for total energy is employed for self-consistent-field calculations.
CaC 2 O 5 s under different pressures are calculated by simultaneously optimizing atomic positions and lattice constants, respectively, using Hellmann-Feynman forces and stresses acting on nuclei and lattice parameters 53 .The phonon mode is determined by finite displacement calculations 54 to ensure molecular stability.Bonding characteristics are determined using Mulliken's population analysis 55,56 .Stress-strain relations calculate elastic constants 23 , where all applied strains have magnitudes of 0.003, and the linear relation was ensured to be sufficient for this strain range.Density of states and partial density of states also caculated by CASTEP code 49 .Crystal structures were visualized with VESTA 57 .

Benchmark calculation
The lattice parameters and cell volume of CaC 2 O 5 s were calculated and compared with reported values to evaluate the accuracy of the density functional theory approach employed here.Differences between our calculated lattice parameters and volume and previous simulation are less than 1.24%, and the difference with experimental results is 2.98% (Table 2), demonstrating the accuracy of our calculation scheme.Compared our calculation results with those of previous researchers under the same pressure, including lattice constant, lattice β Angle and cell volume, Δ is the deviation between the our calculated volume and previous results.
based on the average of the Voight and Reuss moduli.The bulk modulus and shear modulus of CaC 2 O 5 s were calculated and are shown in Fig. 3a, b.The K and G of CaC 2 O 5 s linearly increase with pressure.Among the 6 polymorphs, the K and G of CaC 2 O 5 -Fdd2 are the largest, and those of CaC 2 O 5 -Cc are the smallest.To further explore the effect of pressure on K and G, the pressure derivatives K' and G' were calculated based on linear fitting results of K and G with pressure, respectively.The K' values of CaC 2 O 5 s fall within the range of 3.8 ± 0.3 indicating that the K values of the 6 polymorphs exhibit a similar trend with pressure.The G' of CaC 2 O 5 -C2 has a maximum value of 1.7, and the other 5 polymorphs of CaC 2 O 5 have similar values of 1.2 ± 0.1.K and G of CaC 2 O 5 -I 42d and CaC 2 O 5 -C2-l show very similar values under high pressure, indicating that they have the same elastic properties.

Fig. 2 |
Fig. 2 | Elastic constants of CaC 2 O 5 s.The elastic constants (C 11 , C 12 , C 13 , C 15 , C 22 , C 23 , C 25 , C 33 , C 35 , C 44 , C 46 and C 66 ) of CaC 2 O 5 s at their respective structural phase transition pressures5 were compared (the space group symbol is shown in the figure).Pc, C2, Cc, and C2-l belong to the monoclinic system, with 13 independent

. CaC 2 O 5 -
Cc has the slowest wave velocity, and CaC 2 O 5 -Fdd2 has the largest wave velocity among the 6 polymorphs.As the pressure increases, the wave velocities of CaC 2 O 5 s increase.For 6 CaC 2 O 5 polymorphs, the V S values are sorted as Fdd2 > Pc > C2 > I 42d ≈ C2l > Cc, and the V P values are sorted as Fdd2 > Pc > I 42d ≈ C2-l > C2 > Cc.The wave velocities of CaC 2 O 5 -I 42d and CaC 2 O 5 -C2-l are very close.
2 O 5 -C2-l and CaC 2 O 5 -I 42d are almost the same at the same pressure (the band gap difference is less than 0.02 eV).As the pressure increases, the bandgaps of CaC 2 O 5 -C2-l and CaC 2 O 5 -I 42d gradually increase; however, the bandgaps of the other 4 polymorphs, CaC 2 O 5 -Cc, CaC 2 O 5 -Pc, CaC 2 O 5 -Fdd2, and CaC 2 O 5 -C2, decrease with increasing pressure.Under 10 to 50 GPa, the relative conductivity of CaC 2 O 5 s follows the order CaC 2 O 5 -Cc > CaC 2 O 5 -C2l ≈ CaC 2 O 5 -I 42d; when the pressure is greater than 50 GPa, the relative conductivity of CaC 2 O 5 s is as follows the order CaC 2 O 5 -C2 > CaC 2 O 5 -Fdd2 > CaC 2 O 5 -Pc > CaC 2 O 5 -I 42d.The electronic density of states (DOS) and the band structure of CaC 2 O 5 -I 42d and CaC 2 O 5 -C2-l are nearly identical under the same pressure range (Fig.

Fig. 3 |Fig. 4 |
Fig. 3 | Elastic modulus and elastic wave velocity of CaC 2 O 5 s. a Bulk modulus (solid line) and b shear modulus (point line) of CaC 2 O 5 s (the space group symbol is shown in the figure) within their respective structural phase transformation pressures 5 .c Shear-wave velocity (V S ) and d compressional-wave velocity (V P ) of CaC 2 O 5 s within their respective structural phase transformation pressures 5 .

Fig. 5 |
Fig.5| Shear-wave velocity (V S ) and compressional-wave velocity (V P ) of CaC 2 O 5 s, the PREM model, and some mantle minerals.Comparison of the seismic wave velocity of wadsleyite, ringwoodite, cubic Ca-Pv 31 , MgSiO 3 perovskite18 and CaSiO 3 perovskite17 and CaC 2 O 5 s (the space group symbol is shown in the figure) with that of Earth's mantle according to the PREM22 , the black thin dotted line, the gray area, and the black arrows represent the boundaries at which the phase transition begins, the region where the phase transition occurs, and which two types of CaC 2 O 5 undergo the phase transition5 , respectively.

3 , 4 .2 5 5 RCC 3 R 3 C 3 C C O O 2 2 V V e e l l l o o c c i i i t t y y d d o o w w n n 7 U 3 D
CO 2 and carbon-bearing minerals can also be transported to the Earth's shallow through C C Ca aC C O O --C C2 2 2 Re ea ac ct ti i io on n w wi i it th h M Mg gS Si i iO O a an nd d M Mg gO O Ca aC C O O --I I4 42 2d d & & C C2 2--l l l Ca aS Si i iO O 3 Re ea ac ct ti i io on n w wi i it th h C Ca aS Si i iO O 3 Ca aC CO O 3 Up pw we el l ll l li i in ng g w wi i it th h m m a an nt tl l le e p pl l lu um m e es s M Mg gC CO O 3 De ec co om m p po os si i it ti i io on n 1 1 7 7 0 0 0 0 k k m m L L L L S S V V P P s s C Ca aC C O O --I I4 42 2d d & & C C2 2--l l l 2 2 5 5

Fig. 6 |
Fig. 6 | Schematic diagram of CaC 2 O 5 s in the mantle.Crystal structure of CaC 2 O 5 s and its possible reactions and reaction environments in the mantle are described.Solid arrows indicate the reaction or phase transition that generates CaC 2 O 5 (the red letter part indicates the phase transition conditions 45 ), while dashed arrows indicate the reaction that generates other substances 3 .
Article https://doi.org/10.1038/s41467-024-44925-9Nature Communications | (2024) 15:755 Therefore, CaC 2 O 5 may play a significant role in Earth's deep carbon cycle as follows: CaC 2 O 5 would transports carbon to the deep Earth while it also undergoes successive structural phase transformations with increasing pressure, after which CO 2 and carbon-bearing minerals are produced via reactions of CaC 2 O 5 with minerals that transport carbon to shallow parts of the Earth.The structural, electronic, and elastic properties of CaC 2 O 5 s, including those of CaC 2 O 5 -Cc, CaC 2 O 5 -Fdd2, CaC 2 O 5 -C2-l, CaC 2 O 5 -C2, CaC 2 O 5 -I 42d, and CaC 2 O 5 -Pc, were studied under high pressure by firstprinciples simulation.The calculations show that CaC 2 O 5 is a group of low-density mantle minerals with a low seismic wave velocity.The CaC 2 O 5 -I 42d and CaC 2 O 5 -C2-l exhibit good agreement in their crystal cell structures, electronic state densities, band structures, and seismic wave velocities, indicating that these two polymorphs can be considered the same.Changes in seismic wave velocity caused by phase transformation of the CaC 2 O 5 -Cc to CaC 2 O 5 -I 42d (CaC 2 O 5 -C2-l) agree with the wave velocity discontinuity at a depth of 660 km in the mantle transition zone and maybe one of the origins of this discontinuity.Moreover, when CaC 2 O 5 -Fdd2 transforms into CaC 2 O 5 -C2, its V S decreases by 7.4%, and its density increases by 5.8%, perfectly matching the characteristics of LLSVPs.As the pressure increased, CaC 2 O 5 eventually transformed into CaC 2 O 5 -C2.Generally, CaC 2 O 5 and its high-pressure polymorphs may be among the main components of LLSVPs.

Table 1 |
The plane-wave energy cutoff and k-points settings of CaC 2 O 5

Table 2 |
Structure and volume of CaC 2 O 5 s